A dynamics model of passenger car with seven freedoms considering lateral-load transfer rate was set up to deal with roll stability. The constructed model proves to be rational through vehicle road tests. According to the model, the simulation of steering wheel angle step input with different speeds was conducted, and the factors of roll stability were analyzed by the construction parameters and speed. The simulation result of Matlab shows that the driving axle of the passenger car is the dangerous axle, and when the speed or steering angle exceeds the threshold value it lifts off firstly. The roll stability of the passenger car can be promoted by enlarging track, reducing the center height of sprung mass and enhancing suspension roll stiffness.